Optoelectronic and electronic hybrid integrated circuits are key components of modern telecommunication systems. These circuits typically comprise various circuit elements which are precisely aligned and bonded to a single substrate with high strength, electrically and thermally conductive bonds. Some circuit elements such as laser chips must be aligned to submicron tolerances.
Soldering is the method often used for achieving these conductive bonds. Soldering involves placing a metal mixture between the circuit element and the substrate, melting the mixture to allow it to intimately contact the circuit element and the substrate, and resolidifying the mixture to cause it to bond the circuit element to the substrate. Resolidification of the mixture is typically achieved by lowering the temperature of the circuit element, substrate and mixture. This causes the mixture to freeze and adhere to the circuit element and the substrate. In order to maintain the requisite spatial alignments, each circuit element is individually placed, precisely aligned, and heated to solder the element to the substrate.
Unfortunately, the heat applied during soldering of a subsequent circuit element may cause an earlier soldered circuit element to become misaligned or dislodged because the solder remelts. Solder remelting may also occur during subsequent circuit processing which involves heating of the substrate.
Solder remelting may be reduced or substantially eliminated by using a kinetically controlled solder bond. The assignee herein in U.S. patent application Ser. No. 08/955,686 filed on Oct. 22, 1997, by Coult et al. describes a method and compositions for achieving a kinetically controlled solder bond. Coult et al. uses a binary gold-tin (Au—Sn) solder composition for bonding the circuit elements to the substrate, a quenching layer which freezes the solder bonds produced by the binary solder, and a barrier layer that delays the freezing action of the quenching layer for a selected period of time. The quenching layer comprises one of the two metallic elements of the binary solder and the barrier layer comprises a third metallic element, such as platinum (Pt), which remains a minority component of the mixture. Accordingly, an Au5Sn intermetallic solder bond compound is formed between the circuit elements and the substrate. Because the barrier layer forms only a minority component of the of the final solder bond composition, 5 parts of Au must be used for each part of Sn.
The method and solder compositions of Coult et al. are satisfactory for many applications. However, there are applications that require more efficient use of metallic materials and allow debonding of components from the substrate. Accordingly, there is a need for an improved method and solder compositions for kinetically controlled solder bonding parts which utilize materials more efficiently and permit part debonding.